The present invention relates to a spatial light modulator, an image projector apparatus using the modulator and method of operating the same which can handle high light intensities. More specifically, the present invention relates to a cooling mechanism for efficiently cooling the heat-emitting components of such a projection display apparatus.
Large-screen, high-brightness ( greater than 1000 lumens) electronic projection display apparatuses serve different broad areas of application:
electronic presentations for business, education, advertising,
entertainment, e.g. home theater, electronic cinema.
status and information, e.g. military, utilities, transportation,
simulation in e.g. training and games.
Recently, in response to various limitations of the LCD technologies, high-brightness systems have been developed based on digital light processing (DLP) technology. At the heart of a DLP projection display is provided a spatial light modulator (SLM) unit. A spatial light modulator unit comprises at least one spatial light modulator, which is a device that modulates incident light in a spatial pattern corresponding to an electrical or optical input. The incident light may be modulated in its phase, intensity, polarisation, or direction, and the light modulation may be achieved by a variety of materials exhibiting various electro-optic or magneto-optic effects or by materials that modulate light by surface deformation. An SLM consists of a one- or two-dimensional array of light-modulating elements. Silicon technology used in projection data monitors is capable of producing small-sized, two-dimensional light-valve arrays having several hundred thousand to several million light-modulating elements.
Spatial light modulators are either transmissive or reflective. Transmissive devices modulate the light beam as it passes through the unit. Reflective devices modulate the light as it reflects from a mirror inside the unit.
A deformable mirror device (DMD), also called digital mirror device or digital micro-mirror device, is one embodiment of a reflective SLM, see for example U.S. Pat. No. 5,061,049. It is a semiconductor-based array of fast, reflective digital light switches that precisely control reflection of a light source using, for example, a binary pulse width modulation technique. Combined with image processing, memory, a light source, and optics it forms a DLP system capable of projecting large, bright, seamless, high-contrast color images. A DMD has a matrix of a plurality of individually electrically deformable or moveable mirror cells. In a first state or position, each mirror cell of the deformable mirror device acts as a plane mirror to reflect the light received to one direction (through a lens towards a projection screen for example), while in a second state or position they project the light received to another direction (away from the projection screen).
Reflective spatial light modulators, such as DMDs or reflective LCDs, are heat sensitive, and a hot DMD results a decrease of lifetime and/or in discoloration of the projected image.
A cooling mechanism for cooling the internal parts of a projection display apparatus, is generally built in into it. In general, outside air is introduced into the projector apparatus through an intake opening using an intake fan, and flows over the internal heat-emitting components to cool them. After cooling each of these parts, the air is vented by an exhaust fan to the ambient environment from an air vent opening in an outside case of the projector apparatus. Only one fan can be used for both intake and exhaust.
When air is introduced from outside the apparatus, dust and other foreign matter, such as smoke, oil, may penetrate to the inside of the apparatus. An air filter is generally placed over the intake opening to capture dust and filter the air, but it is still possible for fine particles to penetrate to the inside of the apparatus. When such particles enter the inside of the apparatus with the outside air, they may adhere to the surface of the optical elements. When particulate matter thus adheres to a DMD, it causes decreased light output, or it may hinder the individual mirrors in their movements, resulting in severely deteriorated image quality. A known solution to this problem is to disassemble the projector apparatus and replace the DMOs, as the spaces around a DMD are usually very small, and it is very difficult to clean a DMD in its mounted position.
There has been a long felt need to cool the heat-emitting parts inside the projector apparatus in a way that prevents particulate contamination.
U.S. Pat. No. 6,007,205 describes an LCD projector with an optical lens unit comprising transmission LCD""s and other light transmitting heat-emitting components, such as polarising plates. The optical lens unit is enclosed in a substantially airtight internal chamber. A circulating air current is created by force, by means of a circulation fan, inside this chamber, which cools the heat-emitting components. Preferably means are provided for introducing outside air and for directing the outside air around the outside surface of the members separating the substantially airtight chamber. A heat exchange action between the circulating air current created in the airtight chamber and the outside air flowing through the intake air path is thereby accomplished. In this way, the projector is provided with a cooling mechanism such that no dust or foreign matter penetrates to the inside the optical lens unit.
The LCD projector described has a limited ability to operate at high light intensities. It is difficult to implement effective cooling of the optical components at the highest brightness levels. Particularly for LCD""s which operate as light valves, i.e. they either pass or absorb light, the absorbence of light energy in the LCD for those parts of the projected image which are dark results in large amounts of light energy being absorbed within the sealed chamber (e.g. in the polariser). It is difficult to remove this heat energy by forced convection cooling inside a sealed chamber which relies on an inefficient air-to-air heat exchanger to remove the heat from the sealed chamber and delivery it to an outer chamber.
U.S. Pat. No. 5,170,195 describes methods of cooling transmission LCD""s. These methods involve placing cooling liquid in the light path. It is difficult to guarantee perfect optical clarity of a cooling liquid over the life of a projector. Also, the cooling liquid absorbs some of the transmitted light which reduces the transmitted light intensity and increases the load on the cooling circuit of the LCD. The density of the cooling liquid can change polarisation direction of the light thus causing discolorations of the projected image.
It is an object of the present invention to provide an SLM unit, a projector apparatus and a method of operating the same which can handle high light intensities, without dust or foreign matter penetrating to the sensitive optical components inside of the apparatus and thus deteriorating the quality of the image projected.
According to the present invention, there is provided a spatial light modulating unit for a projector apparatus wherein the spatial light modulating unit is provided with a plurality of SLMs (Spatial Light Modulators, for example, one for each color) and a light splitting unit such as a prismatic device for splitting white light into color components. The spatial light modulating unit also has means for combining the light reflected from the SLMs for projection. This combining device can be the same prismatic unit as used for light splitting. The spatial light modulation unit is enclosed by a substantially sealed chamber. A cooling connection is provided for each SLM, whereby the cooling connection passes through the wall of the substantially sealed chamber and makes contact with each SLM in a heat conductive manner. In this way heat from the main heat sources within the sealed chamber is brought outside the sealed chamber for more efficient cooling, e.g. by an array of fins and a forced convection cooling system. Each SLM is preferably a reflective SLM, such as a DMD or a reflective LCD. With a reflective SLM, the path of light color components of the white light incident on the SLM are preferably remote or distal from the cooling connection for the SLM. Hence, the light needed for projection does not pass through the cooling connection. Particularly preferred are mirror deflection devices such as DMDs which can reflect black light energy out of the chamber. xe2x80x9cBlack lightxe2x80x9d is the difference in light between incident light and the light to be projected, i.e. it is the light energy which must be removed from the incident beam as it is not used for projection of the image on the screen, for instance in parts of the image which need not be illuminated. Preferably, each reflective SLM is located opposite a different face of a prism or prisms which is used to separate white light into component colors. One component of color separated light exits each different face of the prism before it strikes the reflective SLM.
Preferably, the light splitting unit is located substantially centrally in the sealed chamber and the plurality of reflective SLMs are arranged radially about the central light splitting unit, each heat path connection transferring heat in a starting linear direction substantially the same as the direction of light incident on each reflective SLM.
Preferably a forced convective cooling means for cooling the light splitting unit is provided inside the substantially sealed chamber, for example a circulation fan or blower for moving the gas in the sealed chamber. The sealed chamber is usually filled with air at atmospheric pressure. A very uniform temperature of a prism or prisms is desirable in order to have a good image quality.
Additionally, means for additional cooling of the SLMs may be connected to the heat path connection at the outside of the substantially sealed chamber. These means may be, for example, a heatsink, a Peltier element, other electrical cooling means or any other cooling means such as water cooling.
According to another embodiment, means are provided for removing xe2x80x9cblack lightxe2x80x9d out of the substantially sealed chamber so that heat energy within the black light is absorbed outside the sealed chamber.
The present invention also provides a method of cooling a projector apparatus having a spatial light modulating unit provided with a plurality of spatial light modulators (SLMs) and a light splitting unit for splitting white light into color components, the spatial light modulating unit being enclosed by a substantially sealed chamber having walls, the method comprising the step of: cooling each SLM via a heat path connection through a wall of the sealed chamber, the heat path connection being heat conductively connected to each SLM. The heat connection may be passive and comprise a heat conductive connection, for example, massive metal or may be an active cooling system such as forced liquid or gas cooling, heat pipes.
Other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings.